[ ^32 ] 



the orange would in a great meafure vanifh, and the red would 

 appear to extend much farther than in the original fpedrum ; be- 

 caufe the yellow rays being confiderably obftruded, the red would 

 become more predominant ; and that part of the fpedrum, which 

 before appeared orange, in confequence of a certain mixture of 

 yellow and red, would now, by the failure of fo confiderable a part 

 of the yellow, lofe its orange appearance, and put on that of red : 

 and, on experiment, I found the cafe to be fo really in fad; for 

 while an affiftant looked at the fpedrum through the red glafs, 

 I moved an obftacle from the red towards the other end of the 

 fpedrum, defiring him to flop me, when the obflacle fhould arrive 

 at the confines of red and-orange ; but when he did fo, the obftacle 

 had attained the middle of the orange, or rather had paffed 

 beyond it. Now if the orange were really a primitive colour, I 

 ihould fuppofe, that when looked at through the red glafs, it 

 w^ould either appear diluted, without any change of dimenfions j 

 or that if the weak part of the orange, next the red, rtiould va- 

 nifh, by the obftruilion of the glafs, a dark interval would appear 

 between the orange and the red ; in neither cafe can we account 

 for the apparent extenfion of the red into the region of the orange ; 

 nor by any other hypothefis, as appears to me, than that fome of 

 the red rays are equally refrangible with fome of the orange. 



There is another argument derived from the ocular fpedra of 

 Dr. Darwin, which flill further corroborates the dodrine of three 



primogeniaL 



